1. Field of the Invention
The invention relates in general to calibrating a focused beam of energy in a solid freeform fabrication apparatus, and, in particular, to a method of measuring the propagation characteristics of the beam to produce beam propagation data. The beam propagation data can be used to verify that the beam is operating within tolerance, an and/or produce a response that can be used to further calibrate the beam. The invention is particularly useful in determining asymmetric conditions in the beam.
2. Description of the Prior Art
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies can generally be described as Solid Freeform Fabrication, herein referred to as xe2x80x9cSFF.xe2x80x9d Some SFF techniques include, for example, stereolithography, laminated object manufacturing, selective phase area deposition, multi-phase jet solidification, ballistic particle manufacturing, fused deposition modeling, particle deposition, laser sintering, and the like. In SFF, complex parts are-produced from a build material in an additive fashion as opposed to traditional fabrication techniques, which are generally subtractive in nature. For example, in traditional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part. Generally, SFF technologies such as stereolithography, selective laser sintering, and the like, utilize a computer graphic representation of a part and a supply of a build material to fabricate a part in successive layers. The build material is typically a powder, liquid, or paste that is solidified, cured, or sintered when stimulated by a focused beam of energy. Normally, the focused beam of energy is selectively scanned across successive layers of the build material to produce a three-dimensional object. Often, the focused beam of energy used is a high powered laser, such as, for example, an Ultra-Violet generating laser used to cure liquid photopolymer materials.
There are many parameters that must be controlled when utilizing a focused beam of energy in an SFF apparatus. For example, the width of the beam and the intensity of the beam are important characteristics that typically must be precisely controlled in order to produce three-dimensional objects of high quality and consistency. In addition, there must be some process or procedure to track the location of the focused beam and or monitor the condition of the beam. Previous expedients in monitoring a beam can be found in, for example, U.S. Pat. No. 5,267,013 to Spence, which discloses an apparatus and method for obtaining the profile intensity of the beam in a stereolithography machine. The apparatus utilizes a sensor comprising a photodetector located behind a pinhole. The photodetector takes measurements of a laser beam as the beam is moved over the sensor, and a beam intensity profile is produced. The profile provides useful information that is indicative of how the beam will cure a photopolymer material, and the information can be used to optimally select various solidification parameters such as cure width, or the like. Undesirably, however, the information is only two-dimensional and does not clearly indicate the true condition of the laser beam. For example, if the beam has an asymmetric condition such as an astigmatism, the two-dimensional data of the profile is insufficient, by itself, to detect the condition, let alone compensate for it.
Until recently there was no agreed upon standard to characterize a beam. However, the xe2x80x9cM2xe2x80x9d standard for characterizing a beam has recently been adopted by the passing of ISO 11146. As used herein, xe2x80x9cto characterize a beamxe2x80x9d means to obtain sufficient measurements from the beam to be able to map the three-dimensional propagation characteristics of the beam and/or calculate the values of the beam according to the xe2x80x9cM2xe2x80x9d standard. The M2 standard, wherein the M2 value is herein referred to as xe2x80x9cthe times-diffraction-limit number,xe2x80x9d takes into account the threedimensional nature of a focused beam to quantify the propagation characteristics of the beam. Generally the value of M2 is indicative of how close a beam is to an ideal beam. For example an M2 value of 1.0 indicates an ideal beam. M2 values can be calculated from the following equation:
M2=(xcfx80xc3x972xc3x97W0xc3x97"THgr"(4xc3x97xcex)
where W0 is minimum waist radius of the beam, "THgr" is the divergence angle of the beam, and xcex is the wavelength of the beam. However, to obtain these values for a real beam, three-dimensional data must be extracted from the beam. Generally, this requires taking three-dimensional measurements of the beam, not just unlinked two-dimensional profiles. In addition, when a focused beam has an asymmetric condition such as astigmatism, M2 values or calculations must be taken or made in two different directions in order to characterize the condition. One instrument capable of making such measurements and calculations is disclosed in, for example, U.S. Pat. No. 5,267,012 to Sasnett et al. The instrument in Sasnett et al. optically transforms the propagation characteristics of the focused beam prior to taking measurements in the transformed state. The measurements are then processed via extrapolation techniques to finally determine the original propagation characteristics of the beam. Thus, the instrument in Sasnett et al. substantially alters the propagation characteristics of the focused beam prior to taking measurements. Such a device could be permanently mounted in an SFF apparatus to measure beam propagation characteristics; however, to do so is undesirable, as it is a relatively complex and expensive component that would not frequently be used. For instance, the need to completely characterize the focused beam in accord with the M2 standard may only arise once or twice, such as during the assembly of the system in order to assure that it will operate within specification. In addition, it would be desirable to be able to perform such measurements on existing SFF equipment that may not be suited to physically receive the diagnostic device disclosed in Sasnett et al. to make the measurements.
Thus, there is a need to develop a method to characterize a focused beam in an SFF apparatus with existing equipment and without adding additional components. There is also a need to completely characterize a focused beam in an SFF apparatus to produce a response indicative of the condition of the beam. There is also a need to provide a simple and effective method to determine whether a focused beam needs to be replaced, or to change the focus point of the beam in order to compensate for an asymmetric condition found in the beam. In addition, there is a need to completely i characterize an adjustable focused beam in an SFF machine in order to eliminate any asymmetric condition detected in the beam. These and other difficulties of the prior art have been overcome according to the present invention.
The present invention provides its benefits across a broad spectrum of SFF technologies. While the description which follows hereinafter is meant to be representative of a number of such applications, it is not exhaustive. As will be understood, the basic apparatus and methods taught herein can be readily adapted to many uses. It is intended that this specification and the claims appended hereto be accorded a breadth in keeping with the scope and spirit of the invention being disclosed despite what might appear to be limiting language imposed by the requirements of referring to the specific examples disclosed.
It is one aspect of the present invention to provide a simple and effective method to determine the propagation characteristics of a beam of energy in an SFF system.
It is another aspect of the present invention to develop a response to the propagation characteristics determined in a beam of energy in an SFF system.
It is a feature of the present invention to develop a response to the propagation is characteristics determined in a beam of energy in an SFF system that lets the operator know if the condition of the beam is acceptable.
It is another feature of the present invention to analyze the propagation characteristics measured in a beam of energy in an SFF system in order to produce a response to compensate for a non-optimal condition detected in the beam.
It is still another feature of the present invention to analyze the propagation characteristics measured in a beam of energy in an SFF system in order to produce a response to eliminate any non-optimal condition detected in the beam such as an out-of-focus condition, an astigmatic condition, an asymmetrical waist condition, an asymmetrical divergence condition, or any combination thereof.
It is an advantage of the present invention that the propagation characteristics of a beam of energy used in an SFF system can be determined without substantially altering the propagation characteristics of the beam when taking profile measurements.
It is another advantage of the present invention that the propagation characteristics of a beam of energy used in an SFF machine can be determined by utilizing existing spot or slit sensors previously used to produce two-dimensional beam profiles.
These and other aspects, features, and advantages are achieved/attained in the solid freeform fabrication apparatus of the present invention that employ a platform, a laser beam generator, beam conditioning optics, scanning optics, a profiling stage, a sensor, and a controller, all of which are in communication with an apparatus structure. The platform supports the build material of a three-dimensional object when IT the object is formed by the SFF apparatus. The laser beam generator produces the energy that is received by the beam conditioning optics. The beam conditioning optics then transmit the energy in the form of the focused beam of energy to the scanning optics which then direct the focused beam towards the platform. The beam conditioning optics establish the propagation characteristics of the beam. In one embodiment, the beam conditioning optics are adjustable to change the focus point of the beam along the propagation axis of the beam. In another embodiment, the beam conditioning optics are also adjustable so as to allow for the complete adjustment of the propagation characteristics of the beam in multiple directions. The scanning optics are used to rotate the focused beam of energy about a reference position so as to selectively direct the focused beam on the build material located above the platform in order to build three-dimensional objects, as desired. The scanning optics are also used to direct the focused beam on the profiling stage, wherein the measurements are taken to measure and calibrate the focused beam of energy. It is preferred that the scanning optics do not substantially alter the propagation characteristics of the focused beam when scanning the beam across the build material or when directing the focused beam at the profile stage.
The profile stage includes at least one sensor for taking at least two measurements indicative of the width of the focused beam at a minimum of three different planar positions that are orthogonal to the propagation axis of the focused beam. The relative distance between each planar position is tracked by any desirable means so that, along with the measurements indicative of the width of the beam, beam propagation data is produced. Preferably two measurements are taken in each planar position, one measurement being taken in a first direction and the other being taken in a second direction. The first and second directions are mutually perpendicular and symmetrically oriented about the propagation axis of the focused beam of energy. The measurements taken in each planar position and the relative distance between each planar position are provided to the controller which processes them to produce the beam propagation data that characterizes the beam. The beam propagation data is then analyzed to detect a non-optimal condition of the beam, in which a response is produced when a non-optimal condition is detected.
In one embodiment, the response indicates to an operator that the non-optimal condition of the focused beam is unacceptable for use in the apparatus, that is, the condition is beyond an acceptable range for the SFF apparatus. In another embodiment, the response is provided to the beam conditioning optics to adjust the focal position of the beam along the propagation axis to an optimized position taking into account the non-optimal condition detected in the beam. In yet another embodiment, the beam propagation data is delivered to a display device such as a monitor or printer to produce a graphic display of the propagation characteristics of the beam. In still yet another embodiment, the response is delivered to the beam conditioning optics of a laterally adjustable beam of energy that is able to eliminate the non-optimal condition measured in the beam. It is envisioned that any combination of the above embodiments can be used and combined, as desired, depending on the application. For instance, it may be desired or convenient to provide the graphic display of the propagation characteristics of the focused beam in every embodiment.
When processing the measurements it is desirable to complete a number of calculations. For instance, in one embodiment the beam waist in the first direction, the beam waist in the second direction, the first focal point value for the beam waist in the first direction, and the second focal point value for the beam waist in the second direction, are determined. In addition, an astigmatism value can be determined by comparing the first focal point value and the second focal point value. Further, a first divergence angle and second divergence angle of the beam can be determined from the measurements taken in the first and second directions respectively. With these values, a first times-diffraction-limit number and a second times-diffraction-limit number (M2 values) can be determined. Preferably, these values are calculated by the controller and displayed graphically on a monitor, as desired.